Sometimes it is trucks painted in military colours. At other times, men march past dressed in long green greatcoats and fur hats, clutching wooden clubs.

China says it is fighting terrorism and separatism here and must be vigilant.

The security is a constant presence. And for months now Xinjiang's people have had their communication with the outside world restricted.

Internet blamed

China clamped down after Xinjiang suffered serious unrest last summer. The spark was in southern China, where two ethnic Uighurs had been beaten to death by Han Chinese workers after trouble blew up at a factory.

Urumqi was flooded with security forces after the violence

A few days later, after the news reached Xinjiang, came reprisals.

Uighur mobs rioted, attacking Han Chinese, and Han Chinese retaliated. China says 200, mostly Han Chinese, were killed in Urumqi and the internet and mobile phones were used to foment the chaos.

So to keep control in Xinjiang, China imposed severe restrictions on the flow of information in and out of this province.

For much of the past seven months the internet was cut off. Mobile phone text messages were suspended.

Urumqi's biggest internet cafe has 500 computer terminals. It had just been refurbished before the riots with new computers installed.

Now it is almost empty. In one corner three people lie snoozing with their heads on the computer keyboards. The machines are turned off.

Wang Xi, the cafe's manager, says his takings have fallen by two-thirds. His business is just about surviving, he says, but smaller internet cafes are closing.

Even the online computer games that young Chinese love to play were blocked, so a few people sit playing PC games by themselves at a couple of terminals.

"I hope they can switch the internet back on soon," says one. "I've been waiting for a long time to get online."

Xinhua access

More than 20 million people in Xinjiang have been affected by the curbs and most of them had nothing to do with the riots.

Zhu Meng is cut off from e-communications in Urumqi

In Urumqi's wholesale market, traders sit in the snow surrounded by sacks full of dates, nuts and dried fruits. People huddle around charcoal stoves trying to keep warm.

Sifting through a sack of almonds are Ma Hui and her husband Zhu Meng.

The couple make their living trading the fruit and nuts Xinjiang is famed for, selling them to people in other parts of China.

But all their sales are online. So it has been almost impossible for them to keep their business going.

At the computer in his office, Zhu Meng shows me how the authorities have recently been opening up access to a few internet sites.

First he could see only the state-run news agency Xinhua and a couple of other websites. Last weekend curbs on about 25 more sites were lifted, but that is all.

Then he showed me there was no e-mail, no instant messaging, no way of responding to his customers online.

"It's okay for the government to control things," Zhu Meng says, "but when they control everything, even the search engines, it has a huge effect.

"For us who make a living on the internet, it's a fatal blow."

E-mail trek

The only way to get around the internet block has been to travel 1,000 km (620 miles) across Xinjiang's deserts to reach a working internet connection outside the region.

The couple are losing customers

Starting seven months ago, Zhu Meng began making the long journey just to send e-mails to keep his business alive.

The road follows the old trading artery, the Silk Road, past snow-capped peaks, across Xinjiang's empty expanses and through a barren moonscape of mountains and snow.

Over the past seven months, others too have been following the same route by road, train and plane just to get online.

By car, it takes 24 hours from Urumqi to reach the first working internet connection outside Xinjiang. It is just across the border in neighbouring Gansu province, in the dusty frontier town of Liuyuan.

There is one main street, a shabby bus station and an internet cafe.

When Ma Hui and Zhu Meng made the journey again and logged on last week, the cost of being cut off was clear.

They found order after order cancelled. Angry customers who had placed orders but had not received goods were wanting their money back.

"We've lost a lot," said Ma Hui, shocked, "but it's not just the money that's important. We've lost the trust of our customers."

Out in the deserts here is the very end of China's Great Wall. The muddy brown clay bricks crumble into the dust.

Keeping order has long been a Chinese priority. But today, with its curbs on the internet, China's critics say the country again risks walling itself off from the flow of commerce, ideas and progress.

China's government says that is not the case. When asked why internet access in Xinjiang is still so limited, China's foreign ministry spokesman insisted: "The internet is open in China, and governed by our laws".

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